Metal cone for cathode-ray tubes



y 1954 R. D. FAULKNER 2,682,963

- v METAL CONE FOR cA'rHoDE-im TUBES Filed Oct. 8, 1949 INVENTORPatented July 6, 1954 METAL CONE FOR CATHODE-RAY TUBES Richard DaleFaulkner, Lancaster, Pa., assignor to Radio Corporation of America, acorporation of Delaware Application October 8, 1949, Serial No. 120,400

This invention relates to improvements in cathode ray tubes havingcomposite glass and metal envelopes. More particularly, it relates toimprovements in metal shells for the bulb portions of cathode ray tubes.

As is known, there are a number of advantages to be gained by formingthe envelopes of cathode ray tubes as composite metal and glassstructures. One advantage is that such envelopes usually are very muchlighter than all-glass envelopes of equal strength and size. Another isthat since they are made of stock materials such as sheet metal andplate glass, they are much easier to fabricate, particularly in largesizes, than to cast all-glass envelopes. There is a further advantagewhich results from the fact that metal has much higher tensile strengththan glass. It is that the envelope may be formed with a more nearlyflat glass screen since the rim at the large end of the metal cone willbe able .to withstand the considerable tension to which it will besubjected when the envelope is placed under vacuum. This, of course,contrasts with the known fact that all-glass bulbs should be made asnearly spherical as possible so that in the main the glass will besubjected almost exclu sively to compressive forces or, if they are notso made, then very heavy sections of glass must be used around theperiphery of any flattened portion, such as a portion supporting afluorescent screen, so that the walls supporting it can withstand thetension.

However, the manufacture of composite envelopes presents a number of itsown problems and disadvantages. A first problem relates to thecostliness of the metals which must be used. In composite envelopes itis usually necessary to employ special alloys for the metal portions inorder to obtain satisfactory glass-to-metal seals. For example, onespecial alloy known as Kovar is frequently used for certain types ofcomposite envelopes in which it is desirable that the coefficient ofexpansion of the metal portions be very nearly equal to that of glassportions. Similarly, other special alloys are used for other types ofenvelopes, they also being selected usually for their particularco-efficients of expansion. In general most of these special alloys areusually quite expensive as they include high percentages of costlymetals such as chromium or nickel. While this factor may not present tooserious a problem in a great variety of small receiving tubes and thelike in which the amount of metal required is very small, it does in thecase of large screen cathode ray tubes in which the weight of the metalshell frequently is as much as 12 or 15 pounds.

Accordingly, it has been desirable to construct the metal cone for alarge screen cathode ray tube with different wall thicknesses in itsdifferent portions so as not to Waste material Where 8 Claims. (Cl.2202.3)

greatstrength is not needed, i. e., to use different thicknesses ofmaterial according to the forces acting on its diiferentportions. Forexample, it is desirable to use thin material for the mantle of the cone(i. e., its sloping-wall portion) where relatively small and fairlyuniform forces of atmospheric pressure are exerted and to use thickermaterial for its two end portions which are respectively, alarge-diameter flange, in which the arch of screen disc is supportedwhen the envelope is subjected to atmospheric pressure, and asmall-diameter flange to which the neck is fastened. Moreover, in soforming the metal cone, it is necessary to avoid manufacturing processeswhich are as costly as the metal which is saved. For example, it isimpractical to use a process including the steps of separately formingthe mantle (or sloping-wall portion) of thin material and the flanges ofsuitably heavier material and of then welding them together since therequired welds are too costly. Besides, this process entails thepossibility of porosity or air holes and, for a good quality product, itrequires finish-grinding.

Accordingly, it is commonly the practice to utilize a rather inexpensivespinning process by which it has been possible to attain ratios of about2.5 or 3 to 1 between the thicknesses of flange material, i. e., that ofthe stock material, and that of all the mantle portion. In this processthe cone is spun from a circular blank of sheet metal stock of uniformthickness in such a manner that the stock becomes reduced in thicknessin its portions from which the mantle is formed.

Nevertheless cone manufacture according to the prior art has not provento be entirely satisfactory. For one thing, wastage of material inthemantle was not entirely eliminated. The reasons for continued wastagehave been that on the one hand these small thinning ratios do not resultin an adequately thin mantle when one starts with a heavy blank, andthat on the other hand a heavy blank was used for the purpose ofobtaining a rim strong enough to support the screen disc when theenvelope is evacuated. The amount of reduction in thickness which can beproduced by spinning is not unlimited but depends on the steepness ofthe cone, i. e., it be comes greater only as the top angle of the conebecomes smaller. For the wide angle cones which are most suitable forlarge screen kinescopes this reduction is no greater than by a factor ofthe order of 2 or less while if one assumes the use of cones having topangles as small as 30 for other types of cathode ray tubes, then it canbe as great as by a factor of 4. For another thing, this practice hasentailed considerable increase in manufacturing costs due to theoccurrence of numerous pop-outs prior to evacuation. As indicated above,in composite large 3 screen kinescopes the screen disc can be made quitethin. Because of this the disc is subject to noticeable inward bendingunder atmospheric pressure when the envelope is evacuated. This can beobjectionable, even if there is no danger of implosion, as it willintroduce tensions in the glass of the finished tube and in additionwill entail distortion of the glass disc which may have harmful opticaleffects. It has been the practice to solve this problem by using analloy and a glass which have appropriately unequal co-efficients ofexpansion so that differential shrinking after the disc has been sealedto the cone will cause the flange to place the disc under compressionand thereby bend it outward, i. e., in the direction to shorten both itschord and its radius of. curvature, to the end that the subsequentflattening of the disc, which will occur upon evacuation, will restoreit to its original shape. It is after the disc is placed underperipheral compression by the flange and before the envelope isevacuated that the greatest number of popouts occurs.

Accordingly, it is an object of this invention to devise an improvedcone for a cathode ray tube the large end of which includes a flange forcarrying a convex glass screen disc and is strong enough adequately tosupport the perimeter thereof to prevent it from imploding when theenvelope is evacuated even though the thickness of the material in theflange is no more than between 2 to 3 times that of the cone mantlewhile the material of the flange is thin enough not to include anysubstantial excess.

It is a further object of this invention to devise an improvedsub-assembly for a cathode ray tube, which sub-assembly comprises a coneas set forth above and a glass screen disc sealed to the large endthereof in which the cone is so formed that there will be a reducedprobability of a pop-out of the screen disc prior to evacuation.

It is a further object to devise an improved metal cone to be comprisedin the bulb portion of a cathode ray tube and to carry a screen discthereof in which the metal of the cone has a greater co-efficient ofexpansion than the glass of the disc and in which the end of the conewhich carries the disc is so formed that it has adequate structuralstrength to prevent the disc from imploding, when the envelope isevacuated, by supporting the perimeter thereof under compression, eventhough said end of the cone is composed of materialwhich is no more thanbetween 2 to 3 times as thick as the material composing the mantlethereof while the mantle is thin enough to not include any substantialexcess of material.

It is a further object to devise an improved metal cone to be comprisedin the bulb portion of a cathode ray tube and to carry a screen discthereof in which the metal of the cone has a greater co-eflicient ofexpansion than the glass of the disc and in which the end of the conewhich carries the disc is so formed that it has adequate structuralstrength to prevent the disc from imploding, when the envelope isevacuated, by supporting the perimeter thereof under compression, eventhough said end of the cone is composed of material which is no morethan between 2 to 3 times as thick as the material composing the mantlethereof while the mantle is thin enough to not include any substantialexcess of material, anclin which, moreover, said end of the cone is soformed that there will be a reduced probability of a pop-out of thescreen disc during the interval which occurs after the disc is sealed tothe cone and prior to evacuation of the envelope.

Other objects, features and advantages of this invention will beapparent to those skilled in the art from the following detaileddescription of the invention and from the drawing in which:

Figure l is a cross-sectional representation of an embodiment of theinvention;

Figure 2 represents a detail section of one form of the prior art and.assists in disclosing principles underlying the present invention; and

Figure 3 shows an enlarged sectional portion of Figure l to permitcomparison with the showing of Figure 2 in considering the principlesunderlying the present invention.

Figure 1 shows an improved metal cone [0 according to the presentinvention. It comprises a mantle portion ll having a large open end anda small open end and an axis of symmetry passing substantially throughthe centers of the open ends. At the open ends of the mantle portion IIare large and small diameter flanges l2 and I3, respectively. The largediameter flange I2 includes a tapered seat I4 and a lip 15. In theassembly of a composite envelope, a screen disc [6 is joined to thelarge end of the cone It by being placed upon tapered seat M, in whichposition it is surrounded by the lip I5, and by having its edges sealedto the flange l2 by appropriate application of heat to the periphery ofthe disc and the flange. The details of how this is done are no part ofthe present invention, this being also true of such details as the angleof taper 9 of the tapered seat !4. That this is a critical angle is wellknown. This and ways of assembling the screen disc to the metal cone aredescribed in detail in U. S. Patents 2,254,090 and 2,296,307.

In a completed kinescope, the lower portion Ila of the mantle II will besubjected primarily only to the moderate and fairly uniform compressiveforces which are exerted upon it by the atmosphere. For this reason,this portion of the mantle may be made of very thin material and stillhave adequate structural strength. Composite envelopes, in which themantle thickness is of the order of .04", when placed in a compressionchamber, have withstood as much as 60 to 75 lbs. per square inch ofexternal air pressure with the inside of the envelope under hard vacuum.However, the flanges l2 and I3 and the portions of the mantle which areadjacent thereto are subjected to much less uniform and, in some cases,much greater forces. For example, the large diameter flange is subjectedto tension when the atmosphere presses in on the screen disc l5 andtends to flatten it out, and the small flange is subjected to verycomplex strains and stresses whenever a kinescope tube is lifted by itsneck. For these reasons it has been apparent for some time, as wasindicated above, that the cone should be formed employing rather heavysections of material for the flanges.

Some attempts have been made to obtain mantle walls thin enough to notinclude any excess metal by using blanks of quite thin stock to beginwith. While, as was expected, little difficulty was experienced withimplosions of the mantle walls (even under test at four atmospheres),considerable difliculty was experienced with implosions of screen discs.Apparently when the large-diameter flange was made of such thin stock itwas not strong enough to support the periphery of the glass disc undercompression. Therefore it became customary to use heavier stock.

This choice was based on more than mere cost considerations. Anysubstantial susceptibility to implosion represents a highly dangerouscondition which, moreover, due to age fatigue of the glass, increases asthe tube becomes older. However, in the matter of cost the use of theheavier stock has left much to be desired both because of the wastage ofmaterial in the mantle and becauseof the costliness of the numerouspop-outs. The pop-outs wereaccepted'as not involving any danger for theeventual consumer since they almost entirely cease to occur afterevacuation. In the matter of cost the only alternatives which haveseemed available were to try to see if one could reduce the number ofpop-outs by using even heavier stock, and, if this proved success ful,to Weigh any saving attained thereby against the increased wastage ofexpensive alloy, or simply to accept the manufacturing shrinkageoccasioned by pop-outs as cheaper than the use of more metal. Thoughthere was some evidence of a reduced number of pop-outs with thinnercones, it was not considered possible to use them since this wouldresult in a dangerous finished product.

I have discovered that this was an incorrect conclusion. I made tests todetermine if there was any possibility of devising a light cone of suchimproved structural design as simultaneously to be free of excessmaterial in the mantle; to not incur a substantal number of discpopoutsprior to evacuation; and to afford a tube not unduly susceptible toimplosions, i. e., of such improved structural design that it would nolonger be necessary to accept wastage of material and pop-outs, as theprice for avoiding disc implosions. I found that of several influenceswhich take part in causing implosions there are some which can beeliminated or reduced without necessarily increasing the effective totalof those which take part in causing pop-outs, in other words, that thecauses for the two difiiculties are not as closely related as has beensupposed.

Fig. 2 shows a fragmentary view of a cone in which all of the mantle isthin-walled, i. e., in which the mantle is thin all the way up thesloping side of the cone to the inner perimeter of the large-diameterflange I 2. his figure also shows a fragmentary portion of the screendisc 16.

As pointed out above, in a composite largescreen kinescope it ispossible to employ a relatively thin and flat screen disc since themetal flange into which it is sealed can stand considerable tension andtherefore can serve as a circular abutment to retain the flat arch ofthe disc under compression. It is correct to say that in generalpop-outs occur if the glass at or near to the glass-to-metal seal is notstrong'enough to withstand this compression.

It has been supposed that more particularly the reason for the sealbreakage was greater stretching of the top portion of the large-dieameter flange 12 than of its bottom portion, i. e., of its lip l5 thanof its tapered seat l4, so that the outermost portion of the flangetilts axially backward in the direction T about a center of rotationsuch as F, pulling the tapered seat i i away from the seal glass at theperiphery of disc I E. Then,.according to this supposition, upon thefracture of the seal, the compressive forces were suddenly released,allowing the flange to tilt axially forward to its original shape at thesame time projecting the disc out of the cone. Consistently with this,the reduction in pop-outs, which was occasioned by the use of lightermetal, could be explained as the result of a reduction in thedifferential stretching of the lip l5 and the tapered seat M by anincrease in that of the latter and the possibility existed that pop-outsmight also be reduced through the use of thicker metal for reducingdifferential stretching of the lip and the tapered seat by a decrease inthe former.

Another supposition has been that seal breakage would probably increaseif a quite-thin flange were used since excessive stretching thereofmight lead to movement of the seal-glass on the underside of theperiphery of the disc with respect to the adjacent top surface of thetapered seat 14, causing the former to creep on the latter until itsheared away.

I have found instead that the principal reason for pop-outs is to befound in excessive rigidity of the flange, more specifically in itsinability to tilt rather than in any tendency for it to'do so. When anassembly comprising a cone it and a disc 16 is allowed to cool aftersealing, the differential shrinking by which the disc is placed undercompression reduces both its chord and its radius of curvature. Thiscauses the periphery of the disc 16 to tilt in the direction T about afulcrum corresponding to F If the flange l 2 is flexible enough to tiltwith the periphery of the disc a fracture of the seal is unlikely.However, a heavy flange is not able to do so, and the seal-glass at theperiphery of the disc [6 therefore breaks at the fulcrum point.

Although a flange which is thin enough to be flexible will stretch morethan a rigid one, it appears that a great deal of stretching can .bewithstood without fracturing the seal, probably because of the fact thatwhile it is occurring the metal actually is not receding from the glassand therefore the seal is not under tension. How- 'ever, it should beremembered that there must be a limit to this as excessive stretching isa principal reason for disc implosions.

of the large-diameter end of the mantle l i when,

at the same time, it is subjected to atmospheric pressure). If that endof the mantle is thinwalled it will spread out radially in thedirections R and will also exert outward forces on the flange I2. Forthis reason it has proven advantageous to thicken the upper portion ofthe mantle II in the manner shown in Figs. 1 and 3.

If a cone of this type is made of light material, the flange will beflexibleenough to lessen popouts and, since the flange is freed of theburden of acting as a re-inforcement for the largediameter end of themantle, the cone will be strong enough to avert disc implosions. In thisway, the material may be so light that the cone will not include anyexcess of material in all of the mantle portion Ha even if this portionis not thinned to less than one-half the flangematerial thickness by theoperation of spinning.

I have tested under external pressures of between 45 and 60 pounds persquare inch, i. e., of upwards of three atmospheres, evacuated envelopeswhose cones were formed according to the present invention of such thinstock that there was no excess of material in the mantle portion I lawhen the walls thereof were one-half as thick as the large-diameterflange, and I found them to offer satisfactory resistance to discimplosion.

In practice, this improved design has resulted in saving a considerableamount of costly alloy, e. g., about two pounds of metal for the cone ofa 16-inch tube, in addition to a substantial reduction of manufacturinglosses due to pop-outs. In addition, it is expected to permit the use ofa cheaper alloy which contains less chromium and has a higherco-efficient of expansion. The use of this cheaper metal will cause anincrease in the differential shrinkage between the large-diameter flangeand the screen disc and therefore there will be greater compression andoutward bending of the disc. While it is expected that the improved coneshown herein will permit a cone-and-disc assembly to withstand thiscompression without entailing a pop-out, it is obvious that this wouldcertainly not be true if cones of the prior art type were used.

It is still within the scope of the invention to provide the mantleportion l lb with a thickness intermediate the thickness of the mantleportion Ha and the rim portion I2.

The flange [2, by a proper choice of sheet stock, may be flexible enoughto obtain an acceptable reduction of pop-outs even if the stock is notthe thinnest permissible in-so-far as the strength of the mantle portionIla is concerned. Because of this it is possible, where an increasedsafety factor against disc implosion is preferred to maximum economy, touse stock of an intermediate thickness which will afiord a sufiicientlyflexible flange, very strong peripheral support for the disc, and only aslight excess of wallthickness for the mantle.

The improved cone of the present invention makes possible asimplification in a required operation of annealing the cone-and-discassembly after the disc has been scaled into the largediameter flange.According to conventional annealing practices the sub-assembly would beplaced in an annealing oven at a temperature 1 high enough to cause therelease of strains in the glass without softening it; and thereafter thesub-assembly would be cooled nearly all the Way down to room temperatureby gradual equilibrium-cooling in which the temperature of the oven isso gradually reduced (or the sub-assembly is so gradually moved along anoven in which a falling temperature gradient exists) that all portionsof the sub-assembly would drop in temperature uniformly despite theirunequal conductivities, thicknesses, etc. Assuming that the metal has asomewhat greater co-efficient of expansion than the glass, the followingwill take place during equilibrium-cooling: the temperature of thesub-assembly will drop down to the setting-point of the glass withoutthe disc being placed under compression by differential shrinkage sincein that temperature range glass will simply be displaced by thecontracting flange; as the temperature of the sub-assembly drops belowthe setting-point differential shrinkage will gradually place the glassdisc under increased compression bending it outward and shortening itsradius of curvature by reducing its chord.

In the past it has been considered unfeasible to move a structureincluding glass directly from an annealing oven to surroundings at roomtemperature as soon as the glass has been cooled to below thesetting-point. For example, it has been considered objectionable forsub-assemblies of the kind in question because the metal portions, whichare thin and highly conductive, would tend to give up their residualheat very much more rapidly than the glass disc, which is thick andrather non-conductive, so that at least transiently the magnitude ofdifferential shrinking would be very much greater than that intended forthe sub-assembly when all of its parts are at the same (room)temperature and thata resulting temporary excess of compression on thedisc would very much increase the total number of pop-outs. A moregeneral objection to quick removal from an'annealing oven has been thatif, upon its removal from the oven for cooling, the structure is placedon a rack or other support any glass portions which come into contacttherewith will cool more rapidly than the other glass portions and thuspotentially harmful strains may be set up.

However, by actual test and contrary to expectation, I have found thatit is quite feasible to move such a sub-assembly directly from theannealing oven to surroundings at room temperature immediately after ithas cooled below the setting-point of the glass. Moreover I have foundthat this is particularly true if the subassembly includes the improvedcone of the present invention.

It appears that the reasons for the more general objection do not applyif one uses an external cooling rack in which only the metal cone is incontact with the rack. If this is done there is no difiiculty as tostrains being set up at the point of support since such strains in themetal cone, as distinguished from similar strains in glass, are of minorimportance. When the subassembly is so set out to cool the cone affordsan ideal form of carrier for the disc since its fiange carries the discsymmetrically around its entire perimeter.

The following is a suitable way of using this shortened annealingprocess for sub-assemblies for 16-inch cathode ray tubes whose metalcones have the particular dimensions set forth above and are made of analloy including 28% of chrome and 72% of iron and whose glass discs aremade of soda lime silica glass (one form of ordinary window glass) whichis of an inch thick and can be obtained under the trade name Clearlightas manufactured by the Fourco Glass Co.:- immeditaely after thesub-assembly comes from the sealing fires, i. e., when the glass in theperimeter of the disc is strain-free, place it in an annealing ovenwhich is maintained at a temperature of between 535 and 5'75 degreescentigrade, preferably near to 550 degrees; keep it there for at leastfive minutes; and then move it directly to surroundings at roomtemperature. It is neither necessary to vary the oven temperature overthe five-minute period nor to establish a temperature gradient in theoven along a path of travel for the sub-assemblies. If desired, thesub-assembly may be allowed to remain in the oven for longer than thefive minutes and/or a higher initial temperature may be used so long asthe take-out temperature is in the region above mentioned. Ijowever, ifthe initial temperature is above that for take-out it is obvious thateither the temperature of the oven must vary over time or a temperaturegradient must 9 be established within the oven and the sub-assemblymoved in accordance with the direction of the gradient.

While I have indicated the preferred embodiments of my invention ofwhich I am now aware and have also indicated certain specificapplications for which my invention may be employed, it will be apparentthat my invention is by no means limited to the exact forms illustratedor uses indicated, but that many variations may be made in theparticular structure used and the purpose for which it is employedwithout departing from the scope of my invention as set forth in theappended claims.

What I claim as new is:

1. A metal shell for a cathode ray tube envelope comprising as a unitarystructure a frustoconical mantle having a substantially uniform wallthickness, a large-diameter flange at the large end of said shell havinga wall thickness which is greater than that of said mantle, said mantleincluding a peripheral portion directly interconnecting saidlarge-diameter flange and said mantle and having a thicknessintermediate the thicknesses of said flange and said mantle.

2. A metal cone for a cathode ray tube envelope comprising as a unitarystructure a frustoconical mantle including a mantle-portion adjacent thesmaller end having a substantially uniform wall thickness, alarge-diameter flange at l the larger end of said cone having a wallthickness which is greater than said mantle, said mantle also comprisinga peripheral portion adjacent said flange having a thicknesssubstantially equal to the thickness of the large-diameter flange anddirectly attached to the large-diameter flange, said peripheral mantleportion extending an appreciable distance from said flange.

3. A cathode ray tube having an envelope comprising a metal shellincluding a frusto-conical mantle, a large-diameter flange attached tothe large end of the mantle in which the large-diameter flange has awall thickness greater than the main portion of the mantle, the portionof the mantle adjacent to and extending an appreciable distance fromsaid large-diameter flange having substantially the same thickness assaid flange, a glass screen disc having its periphery sealed into thelarge-diameter flange, each surface of the disc having an approximatelyspherical curvature, and the disc being retained under compressionwithin the large-diameter flange.

4. A cathode ray tube having a composite glass and metal envelope, themetal portion of said envelope comprising as a unitary structure a shellhaving a small open end and a large open end and an intermediate mantlewith an axis of symmetry passing through said open ends, said mantleincluding a first mantle-portion at the smaller end thereof and having asubstantially uniform wall thickness, a second mantle-portion at thelarger end of the mantle and having a wall thickness greater than thatof said first mantle-portion, a large flange formed at the large end ofsaid mantle and extending outwardly from the outer surface of said shellat a small angle with the perpendicular to said axis of symmetry, saidflange having a wall thickness of the same order as that of said secondmantle-portion, and a glass screen andclosure member sealed at its edgesto said flange.

5. A cathode ray tube having a composite glass and metal envelope, themetal portion of said envelope including a metal cone, thesmall-diameter portion of said cone having a substantially uniform wallthickness, a large-diameter flange formed at the large end of said coneand having a wall thickness greater than that of the small diameterportion, the portion of said cone adjacent said flange being of the sameorder of thickness as said flange, said flange comprising a portioninclined at almost a right angle to a plane transverse of thelongitudinal axis of said cone and a second portion inclined at anobtuse angle to said plane, and a glass screen closure member sealed tothe inside portions of each of said inclined portions of said flange.

6. A metal cone for a cathode ray tube envelope comprising a unitarystructure having a frustoconical mantle including a mantle portion ad--jacent the smaller end of said cone having a substantially uniform wallthickness, a largediameter flange at the larger end of said cone havinga wall thickness which is greater by a factor between 2 and 3 than thethickness of said mantle portion adjacent the smaller end, said mantlecomprising a peripheral portion adjacent said flange having a thicknesssubstantially equal to the thickness of the large-diameter fiange'anddirectly attached to said large-diameter flange.

7. A metal cone for a cathode ray tube envelope comprising a unitarystructure having a frustoconical mantle including a mantle portionadjacent the smaller end of said cone having a substantially uniformwall thickness, a large-diameter flange at the larger end of said conehaving a wall thickness which is greater by a factor between 2 and 3than the thickness of said mantle portion adjacent the smaller end, saidmantle comprising a peripheral portion adjacent said flange having athickness substantially equal to, the thickness of the large-diameterflange and directly attached tosaid large-diameter flange, said flangeincluding one portion forming on its inside an obtuse angle to a planetransverse of the longitudinal axis of the cone and directly joiningsaid mantle and a second portion forming on its inside almost a rightangle with said plane.

8. A metal shell fora cathode ray tube envelope, said shell comprisingas a unitary structure, a tubular mantle member including a largeopening at one end and a smaller opening at the other end thereof and anaxis of symmetry passing through said openings, said mantle portionhaving a substantially uniform wall thickness, a large flange at thelarge end of said shell extending outwardly from the outer surface ofsaid shell at a small angle with the perpendicular to said axis ofsymmetry, said flange having a wall thickness greater than that of saidmantle, said mantle portion also including a peripheral portion adjacentsaid flange having a thickness substantially equal to the thickness ofsaid flange and directly attached to said flange, said peripheral mantleportion extending an appreciable distance from said flange.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 151,435 Ripley May 23, 1874 1,184,813 Birdsall May 30, 19161,922,087 Hiester Aug. 15, 1933 1,939,356 Lingren Dec. 12, 19331,963,008 Weeks June 12, 1934 2,189,261 Bowie Feb. 6, 1940 2,254,090Power Aug. 26, 1941 2,296,579 Seelen Sept. 22, 1942

